1. Field of the Invention
The present invention relates to apparatus and process for the preparation of silane, and, more especially, for the continuous preparation of silane.
2. Description of the Prior Art
It is known to this art to prepare silane (SiH.sub.4) by reacting chlorosilanes having the formula SiH.sub.4-x Cl.sub.x, wherein x is advantageously 3 or 4, with lithium hydride (LiH), and such reaction has been described, for example, in U.S. Pat. No. 3,078,218 and French Pat. No. 2,365,518. The subject chemical reaction employed may thus be represented as: EQU SiH.sub.4-x Cl.sub.x +x LiH.fwdarw.SiH.sub.4 +x LiCl
The lithium hydride starting material is provided by reacting metallic lithium with hydrogen, with the lithium metal itself produced by the electrolysis of lithium chloride. According to the process described in the aforenoted patents, it is known to carry out each of the electrolysis, hydrogenation and silane production reactions in the same medium consisting of molten salts selected from among alkali and alkaline earth metal chlorides, one of which being lithium chloride.
According to such process, the different reactions are conducted in a single, high capacity apparatus, wherein the reaction medium itself plays the dual role of both supplying the reactant (lithium chloride) and also acts as a thermal flywheel, or heat exchanger, to eliminate the heat generated by the different chemical reactions. However, and this is one of the principal disadvantages of the process immediately above-described, the reaction medium which contains the lithium chloride must be used in very large amounts with respect to the amount of lithium chloride (reactant) which is converted in the different reactions. Furthermore, the use of a single apparatus in which the reaction medium also acts as a thermal flywheel does not permit carrying out each of the respective reactions under optimum conditions.
Also known to the prior art, as described in Japanese Kokai No. 56.169120 (Application No. 55.72789), is a process similar to that in the U.S. Pat. No. 3,078,218, and according to which the different reactions are again carried out in a single apparatus, but in which the lithium produced by electrolysis, instead of being immediately hydrogenated upon formation thereof, is separated from the medium of molten salts and transferred into the hydrogenation zone. The process described in this patent has the same disadvantages as that described in said U.S. Pat. No. 3,078,218. Furthermore, the means for transferring product from the reaction medium of one reaction zone to another are very complex and difficult to utilize on an industrial scale. In addition, such single devices render the separation of the chlorine and the hydrogen quite difficult, a separation that is absolutely necessary for the safe use of industrial facilities.
Also known to this art, from U.S. Pat. No. 3,163,590, is a process for the preparation of silane by the same reactions as those described in said '218 patent, wherein the various reactions are effected in a single apparatus, but in separate zones thereof.
Consistent with the embodiments described in said '590 patent, and as illustrated in FIGS. 2 and 4 thereof, it is necessary to separate the lithium metal produced by the electrolysis from the mixture of molten salts prior to its hydrogenation. Furthermore, all of the reactions employed in the process of this patent must be carried out simultaneously, without consideration of their respective kinetics, which are very different, the result being a highly delicate operation of the totality of such reactions, especially in the transition stages of startup, shutdown and variations in production.